1. Field of the Invention
The present invention relates to multiphase protection circuitry for electrical power supply networks, and more particularly to such protection circuitry provided with means for temporarily changing the functional level of the remaining intact phases, after a fault in a phase which causes a temporary disconnection of the faulty phase, in such a way that the resultant phenomena occurring in the intact phases during the switching operation in the faulty phase does not cause disconnection of the intact phases, but, if a real fault occurs in the intact phases, a triple-pole tripping is effected.
2. Prior Art
When a fault occurs in an electrical power supply network, an electrical transient wave propagates therein and, because of repeated reflections in various discontinuities in the network, such as transformers, stations and the sources of the fault, the currents and voltages contain transients which are disadvantageous for the measuring relays in the relay protection devices. The dominant frequencies in such transients depend on the distance to the fault and the network configuration. Investigations have shown that, even with the introduction of transient suppression filters, it is difficult to use, for example, conventional impedance relays for relay protection devices, the operating time of which is to be less than the time of a cycle. In case of higher system voltages, greater demands are placed on the rapidity of the relay protection devices, while at the same time the damping of the transient waves decreases. It is therefore more difficult to apply traditional measuring principles to the relay protection devices.
From, for example U.S. Pat. No. 3,956,671, it is known to utilize the direction of movement of the transient waves at a measuring point for determining the direction to the source of disturbance. By comparing the polarities of the current and voltage waves, for example, it is possible to determine whether the fault lies ahead or behind the measuring point.
In the case of single-phase faults in networks with high system voltages, particularly in radial or thinly meshed networks, it is sometimes desirable for the relay protection devices to release only the faulty phase (so-called single-pole tripping). Since the other two phases are intact, it is easier to maintain stability in the network, at least for some time until high speed reclosing of the originally faulty phase.
In conjunction with the disconnection of a single-phase fault, it may however happen that the transient waves which then occur in the intact phases have a greater amplitude than those which occur in connection with the fault. The magnitude of the transient waves is dependent on the relationship between the load current and the fault current. This relationship may vary a great deal depending on the system voltage, the network configuration, the type of the fault, the source of the fault, and so on.
It is clear from the above that a protection device based on measuring transient waves may sometimes encounter problems with a correct choice of phases in case of a single-phase fault, since disconnection of a single-phase fault may result in a triple-pole tripping. One solution to this problem is to prevent new signals from arriving into the protection device after a single-phase fault has been detected, for example by fully blocking a level detector. However, this is not a very good solution, since as a rule it is desirable to be able to effect triple-pole tripping if a new fault- single-phase or double-phase--occurs in the intact phases before the line circuit-breaker has time to release the first occurring single-phase fault. The tripping time of the circuit-breaker may vary from about 20 ms to 100 ms depending on the type of circuit-breaker used.